1. Field of the Invention
The present invention relates to modems used in telecommunications and more specifically to a DSL modem which is compatible with home networks such as those based on home phone networking alliance (HPNA) standard.
2. Related Art
Digital subscriber line (DSL) technology is often used to provide high bandwidth connections to homes using local loops. Typically, each home contains what is commonly referred to as a customer premises equipment (CPE). The CPE may in turn contain (or interface with) a modem which receives (and transmits) signals representing digital data stream (a sequence of bits typically) encoded according to the ADSL specifications.
A modem may be connected to a telephone line, with potentially several telephone lines together providing connectivity to devices such as CPEs, telephone sets and facsimile machines. Usually, a signal (sent by a remote device) received on a local loop causes a corresponding signal to be transmitted on a telephone line and the receiving devices thus receive the signal on the telephone lines.
The devices using the telephone lines and the local loops are often designed to operate in different frequency bands such that the same medium (i.e., local loops and telephone lines) can be shared by many types of devices using frequency division multiplexing. For example, the telephone sets and facsimile machines operate in the 0-4 kHz band, a CPE transmitting according to ADSL (asymmetric DSL) uses 30-138 KHZ band to transmit upstream (i.e., from home to central office) and 180 kHz -1.1 MHZ band to receive downstream.
Accordingly, an ADSL modem (or an analog to digital converter (ADC) in the modem) is typically designed to sample a received signal in the 180 kHz -1.1 MHZ band, and the resulting samples may be analyzed to recover the data encoded according to ADSL specifications. Problems may be posed to the data recovery task when the telephone lines are shared for other purposes. As is well known, telephone lines are also used to provide connectivity between devices (such as computer systems) using specifications such as those developed by HPNA.
One problem resulting from the sharing of the telephone lines by home networking and DSL technologies, is that HPNA compatible transmissions are in the 4-10 MHZ band, and the signals in 4-10 MHZ band would alias into 180 KHZ -1.1 MHZ band. As is well known, such aliasing would lead to inaccurate recovery of the data unless a corrective action is taken.
The problem may be exacerbated by the fact that the HPNA signals are relatively strong (voltage swing of 0-2 Volts typically) compared to the ADSL related signals (few hundred milli-volts) particularly in the case of long local loops. As an illustration, if the ADSL signals component is desired to be amplified by a certain degree before sampling by an ADC, the ADC would need to operate at a fairly high voltage swing (due to the higher voltage strength of the HPNA signals) if the ADC were to sample the combined HPNA and ADSL signals. Operating at such high voltage swing may be undesirable in many environments at least due to the higher electrical power requirements. While the problem is described above with respect to ADSL for illustration, similar problems may be encountered with other types of DSL technologies also.
Accordingly, what is needed is a method and apparatus which enables a DSL modem to operate in conjunction with home networking technologies such as those based on HPNA.
An aspect of the present invention provides a filter which can be used in a ADSL modem (or CPE in general) to filter the undesirable components such that the filtered component is suitable for subsequent sampling. The filtered signal may be amplified by a suitable factor prior to the sampling.
In one embodiment, the filter is implemented as an analog bandpass filter which attenuates the HPNA signals to less than xe2x88x92150 dBm/Hz. The number xe2x88x92150 dBm/Hz represents the desired noise floor for the ADSL signals in several environments. Attenuation to such low desired levels using analog components may lead to solutions which require high electrical power, and may thus be undesirable.
Accordingly, in an alternative embodiment, the filter is implemented to contain a high pass filter, an amplifier and a low pass filter. The high pass filter filters the low frequency components such as the ADSL transmit echo signal and the telephone voice signal. The output of the high pass filter is amplified and then provided to a low pass filter, which filters the undesirable high frequency components such as the HPNA signals. Due to the prior amplification, the noise requirements of the low pass filter are reduced and the implementation is simplified.
According to yet another aspect of the present invention, the high pass filter is implemented to limit the high frequencies also to some extent. A resistor may be added in series with an input capacitance (of the high pass filter). The resistor attenuates the high frequency signals (including HPNA related) to some extent. When subsequent components are designed to operate with a desired voltage level ceiling, the attenuation (by the resistor) allows for greater amplification in such subsequent components. Specifically, in the described embodiments, the output signals of the resistors (or the high pass filter) may be amplified to a greater extent prior to passing through the low pass filter, thereby further simplifying the implementation of the low pass filter.
According to one more aspect of the present invention, an equalizer may be included between the high pass filter and the low pass filter. The equalizer compensates for the difference of attenuations different frequency components are subjected to when transmitted on local loops. The output of the low pass filter is sampled to generate multiple samples at a high frequency. The samples may be examined later to recover the data encoded in the ADSL signal component.
According to yet another aspect of the present invention, the entire filter may be provided as one monolithic integrated circuit. Such a degree of integration may be enabled by the other aspects of the present invention noted above. The filter may be integrated with other components of a CPE also. In general, implementation as a single monolithic integrated circuit leads to lower costs and may be important at least in consumer type environments.
Therefore, the present invention may be used in ADSL type environments as a filter can be provided which filters the undesirable components while minimizing power and space requirements.
The present invention can be used to minimize the power requirements as a high pass filter can first filter the low frequency components and attenuate to a limited extent the higher frequency components.
The present invention can be used to simplify the implementation of a low pass filter as the signal can be amplified substantially in prior stages due to the operation of a high pass filter.
The present invention can be used to minimize the overall cost of implementation as the filter and other components of a modem/CPE can be implemented as a single (monolithic) integrated circuit.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.